The sugar derivative maltol is a hydroxypyrone (IUPAC name: 3-hydroxy-2-methyl-4H-pyran-4-one) and it strongly chelates iron and the resulting complex (ferric trimaltol which may also be written as ferric tri-maltol) is well absorbed, unlike many other ferric iron therapies. Ferric trimaltol appears well tolerated even in populations highly susceptible to gastrointestinal side-effects, such as IBD patients (Harvey et al., 1998), and as such it provides a valuable alternative to patients who are intolerant of oral ferrous iron products, notably in place of intravenous iron. Clinical trials using ferric trimaltol have been carried out, see for example, Gasche et al., 2015.
However, despite the evidence of bioavailability and tolerability for ferric trimaltol, its clinical development has been limited by the absence of adequate synthetic routes. In particular, most manufacturing processes require the use of organic solvents, which increase manufacturing costs, for example to deal with post-synthesis solvent removal, and require additional safety measures, for example to deal with flammability. Critically, solvent-based syntheses are not robust and often generate ferric hydroxide, described in the prior art to be an unwanted impurity of the synthesis.
WO 03/097627 (Vitra Pharmaceuticals Limited) describes the synthesis of ferric trimaltol from iron salts of carboxylic acids in aqueous solution at a pH greater than 7. In a first synthesis, ferric citrate is added to a solution of sodium hydroxide at room temperature and maltol is added to a second solution of sodium hydroxide at pH 11.6. The ferric citrate solution is added to the maltol solution, leading to the production of a deep red precipitate. This composition is then evaporated until dryness and the material is powdered and dried. Alternative syntheses are described using ferrous fumarate or ferrous gluconate as the iron carboxylate salt starting material, and by dissolving maltol in sodium carbonate solution in place of sodium hydroxide. However, despite the fact that this process is fully aqueous, several of the iron carboxylate salts employed are expensive, especially as they need to be pharmaceutical grade if the ferric trimaltol is to be suitable for human administration. More importantly, this process introduces high levels of carboxylates (equimolar to iron or greater) to the synthesis that are not easily removed by filtration or centrifugation of the ferric trimaltol cake. Instead these water soluble contaminants must be washed off (e.g. water washed), but this would result in considerable losses of the product due to the amphipathic nature of ferric trimaltol.
WO 2012/101442 (Iron Therapeutic Holdings AG) describes the synthesis of ferric trimaltol by reacting maltol and a non-carboxylate iron salt in an aqueous solution at alkaline pH. However, despite the lower cost of non-carboxylate iron salts, pharmaceutically appropriate grades are still required if the ferric trimaltol is to be suitable for human administration and hence are comparatively expensive starting materials. Importantly, the use of non-carboxylate iron salts (e.g. ferric chloride) results in the addition of considerable levels of the respective counter-anion (e.g. three moles of chloride per every mole of iron) of which a significant part is retained in the filtration (or centrifugation) cake and thus must be washed off. As such, WO 2012/101442 does not address the problem of product losses in WO 03/097627. Furthermore, the addition of a non-carboxylate iron salt (e.g. ferric chloride) to a very alkaline solution, as described in WO 2012/101442, promotes the formation of stable iron oxides, which is an unwanted contaminant in ferric trimaltol. As a consequence, further costly and time-consuming processing of the material would be required for manufacturing.
Overall, the cost of the current aqueous syntheses is driven by regulatory demands for low levels of toxic heavy metals and residual reagents in the final pharmaceutical formulation, which force the use of highly purified, and thus expensive, iron salts as well as thorough washing of the final product (resulting in significant losses of product). This will impact on the final price of ferric trimaltol and potentially limits patient access to this therapy. As such, there is a need for a process that can use lower iron grades and limited wash cycles, whilst producing ferric trimaltol of adequate purity.
Accordingly, it remains a problem in the art to provide processes for the synthesis of ferric trimaltol at economic cost and which overcome some or all of the drawbacks set out above that are associated with prior art. Solving these issues through better synthesis of the material would allow good patient access to ferric trimaltol.